Photolithographic mask

ABSTRACT

The invention provides a photolithographic mask for the exposure of radiation-sensitive resist layers on semi-conductor substrates, the mask having at least one radiation-transmissive substrate and at least one radiation-opaque layer and/or at least one half-tone layer. The radiation-opaque layer and/or the half-tone layer are used to provide main features, the main features being formed in such a way that the pattern formed by the main features is transferred into the resist layer when irradiated, and the radiation-opaque layer and/or the half-tone layer are used to provide assist features, the assist features being formed in such a way that the pattern formed by the assist features is not transferred into the resist layer when irradiated.

[0001] The present invention relates to photolithographic masks. Thepresent invention relates, in particular, to photolithographic masks forthe patterning of radiation-sensitive resist layers on semiconductorsubstrates for the fabrication of large scale integrated semiconductorcomponents.

[0002] In the course of the ever decreasing structure dimensions for theproduction of large scale integrated semiconductor components, adimensionally accurate photolithographic transfer of mask structures toradiation-sensitive resist layers becomes more and more important. Inthe meantime, semiconductor components are fabricated with structureline widths of 180 nm or less for commercial use in large volumes, sothat the requirements made of the patterning process steps must satisfyvery high standards. In this case, the photolithographic transfer ofmask structures to radiation-sensitive resist layers is one of theoutstanding techniques for patterning layers on semiconductorcomponents.

[0003] In this case, the photolithographic transfer of mask structuresto a radiation-sensitive resist layer is effected in a plurality ofsteps. The alignment of the mask above the substrate covered with theradiation-sensitive resist layer is followed by the exposure of theradiation-sensitive resist layer through the mask for marking the resistlayer material to be removed (or to be left). In this case, the exposureof the radiation-sensitive resist layer can be effected in thesilhouette method, the mask bearing on the resist layer (contactexposure) or being applied closely above the resist layer (proximityexposure). For very high resolution patterning, on the other hand, theexposure is carried out by means of a so-called projection exposure. Inthis case, the light that has passed through the mask is focused in aprojection objective onto the resist layer, the projection objectiveimaging the maximum possible number of higher orders of diffractionproduced by the mask structure. This imaging method makes it possible toimage a minimum transferable structure line width bmin of

b _(min) =k ₁(λ/NA)  (1)

[0004] from the mask onto the resist layer. In this case, λ is thewavelength with which exposure is effected, and NA is the numericalaperture, i.e. essentially the ratio of half the lens window diameter tothe distance between wafer and lens; in the region of the resolutionlimit, the proportionality constant k₁ adopts values k₁<0.5, and specialmeasures have to be taken to increase the contrast, in order to ensure asufficient process window for the lithography.

[0005] If the radiation-sensitive resist layer is a positive resistlayer, then the exposure brings about at the exposed locations achemical alteration of the resist layer material, which can be flushedout from the resist layer during development. By contrast, if theradiation-sensitive resist layer is a negative resist layer, then thenon-exposed material is flushed out during development. In order toobtain the same structure as in the case of the positive resist, themask must be patterned essentially complementarily with respect to themask for the positive resist.

[0006] The exposure and further steps, such as the initiation of the“PAG” (photo acid generator), the “PEB” (post exposure bake) and theadjustment of the diffusion gradient and therefore of the resistprofile, is followed by the development of the resist layer by sprayingor dripping on developer liquid which selectively strips away (orselectively leaves resistant) the marked resist layer material. Afterthe drying of the substrate, the patterned resist is finally obtained,which, in conclusion, is generally subjected to a thermal step forcuring.

[0007] At the end, the minimum structure line width on the mask which isactually produced after the production of the resist structure isgreater than that calculated from (1), for a number of reasons. Firstly,the resist layer has a finite thickness, so that the imaging blursslightly; furthermore, the developer acts isotropically, so that theresist is also removed in the lateral direction during the developmentof the resist layer. The minimum structure line width on the mask whichis required for the production of a resist layer structure on asemiconductor substrate therefore depends on many parameters and isdetermined individually for each patterning process.

[0008] The mask comprises e.g. an unpatterned quartz glass substratewhich is light-transmissive even in the UV region and on which a thinopaque layer, usually made of black chromium, is applied. The blackchromium layer produces, together with the transmissive regions, themask structure which is imaged onto the resist layer. In this case, theblack chromium layer produces the darkened regions on the resist layer,while the light-transmissive region produces the exposed regions on theresist. If the resist is positive, the rate at which the resist isremoved in the developer is drastically increased in the exposed regionscompared to the unexposed regions and the development step leads to theremoval of material. If the resist is negative, the resist crosslinks inthe exposed regions, so that predominantly the unexposed regions areremoved during the development. Furthermore, for dimensionally accuratefeature transfer, it is important to ensure a homogeneous exposure doseover the region to be exposed.

[0009] Various effects can contribute to impairing the dimensionalfidelity. Firstly, the finite resist contrast γ, which is a measure ofthe resist removal gradient for a given exposure dose, causes roundingof originally cornered mask structures. Furthermore, interferenceeffects, diffraction effects and scattered light which arise atstructure elements of the mask, the resist layer and/or the prepatternedsubstrate surface can result in the effective exposure dose not beinghomogeneous in the resist layer regions.

[0010] In particular the defraction and interference effects of maskfeatures which lie close together, known as proximity effects, cansignificantly impair the dimensional accuracy which can be achieved. Thecloser together the features lie, the more pronounced the proximityeffects become. The result of this, for example, is that features whichshould actually be of the same size are reproduced differently in theresist layer according to what surrounds them. This difference isparticularly evident between features which are arranged very closetogether and features which are substantially isolated without adjacentfeatures.

[0011] To substantially compensate for this difference, it is customaryto use assist features, known as scattering bars or SRAFs=sub resolutionassist features, which are arranged in the vicinity of isolatedfeatures. Accordingly, a structure which is actually isolated now has asurrounding area which substantially corresponds to the area surroundingtightly packed features, so that substantially the same reproductionproperties result. These assist features are formed on the mask in sucha way that they are not themselves reproduced in the resist layer andthey are in each case arranged in parallel to the edges of the actualfeatures on the mask. Assist features of this type are described, forexample, in U.S. Pat. Nos. 5,242,770 and 5,821,014.

[0012] The conventional assist features are particularly suitable forimproving the reproduction properties of isolated features. However, itis quite possible that in the layout of a mask there may be featureswhich are neither clearly isolated nor arranged particularly closetogether. For example, it is not always possible to make the distancebetween two gate paths so great that their associated assist features ineach case fit between the gate tracks, or for the gate tracks to be laidso close together that they have sufficiently good reproductionproperties even without assist features. For these cases, a singleassist feature is generally placed in the center between the two gatetracks. However, this assist feature is generally not at the optimumdistance from the gate tracks, and for many distance ranges it hasscarcely any further enlarging effect on the focus process window inlithography. However, if these distance ranges are prohibited in design,this has adverse effects on design outlay and the chip size.

[0013] Therefore, the present invention is based on the object ofproviding a photolithographic mask which reduces or completely avoidsthe problems described.

[0014] This object is achieved by the photolithographic mask accordingto the invention as described in the independent patent claim 1. Furtheradvantageous embodiments, configurations and aspects of the presentinvention will emerge from the dependent patent claims, the descriptionand the appended drawings.

[0015] According to the invention, a photolithographic mask for theexposure of radiation-sensitive resist layers on semiconductorsubstrates is provided, the mask having at least oneradiation-transmissive substrate and at least one radiation-opaque layerand/or at least one half-tone layer. The radiation-opaque layer and/orthe half-tone layer are used to provide main features, the main featuresbeing formed in such a way that the pattern formed by the main featuresis transferred into the resist layer when irradiated, and theradiation-opaque layer and/or the half-tone layer are used to provideassist features, the assist features being formed in such a way that thepattern formed by the assist features is not transferred into the resistlayer when irradiated.

[0016] In this context, a half-tone layer is understood as meaning alayer which transmits a certain percentage of the radiation and whichshifts the phase of the radiation passing through it by a predeterminedamount. Depending on the particular application, the main or assistfeatures may be formed from subregions of the radiation-opaque layer orof the half-tone layer or as openings in the radiation-opaque layer orthe half-tone layer.

[0017] The photolithographic mask according to the invention ischaracterized in that for one main feature, which is oriented in a firstdirection at least in the region of a partial piece, there are at leasttwo assist features (15), which, adjacent to the partial piece, areoriented in a second direction which is substantially perpendicular tothe first direction.

[0018] According to a preferred embodiment of the photomask according tothe invention, between two directly adjacent main features, which atleast in the region of a partial piece are oriented substantiallyparallel in a first direction, there are at least two assist features,which are oriented in a second direction substantially perpendicular tothe first direction.

[0019] The main features or the partial pieces of which the mainfeatures are often composed are generally of a shape which has aconsiderably greater extent along one direction (longitudinal direction)than in a direction perpendicular thereto. The main features or theirpartial pieces typically have a shape which is similar to a rectangleand is considerably longer than it is wide. Accordingly, it could besaid that the main features or the partial pieces of the main featuresare oriented in one direction, namely their longitudinal direction. Theassist features are generally also of a shape which has a considerablygreater extent along one direction (longitudinal direction) than in adirection perpendicular thereto. Accordingly, the assist features arealso oriented in one direction, namely their longitudinal direction.

[0020] The prior art in each case uses assist features which areoriented substantially parallel to the main features or to partialpieces of the main features. However, as has already been explained,this leads to difficulties in particular in situations in which the mainfeatures are at a distance from one another which is neitherparticularly great (isolated main features) nor particularly small(densely packed main features).

[0021] By contrast, the photolithographic mask according to theinvention uses assist features which are oriented substantiallyperpendicular to the main features. The photolithographic mask accordingto the invention has the advantage that now even main features or edgesof main features whose reproduction properties it was hithertoimpossible to improve by means of assist features experience animprovement in their reproduction properties, with the result that inparticular the process window for the entire lithography process isimproved.

[0022] According to a preferred embodiment of the photolitho-graphicmask according to the invention, the assist features are rectangular inshape. Furthermore, it is preferable if the width b of the assistfeatures lies in the range λ/(10 NA)<b<λ/(3 NA), where λ is thewavelength of the radiation used to irradiate the mask or the resistlayer.

[0023] According to a further preferred embodiment of thephotolithographic mask according to the invention, the distance Dbetween the main features lies in the range λ/(2 NA)<D<3×/NA.Furthermore, it is preferable if the distance x between the assistfeature ends and the main features lies in the range λ/(5 NA)<x<2λ/(3NA) and if the distance d between the assist feature ends lies in therange λ/(3 NA)<d<3λ/(2 NA).

[0024] According to a further preferred embodiment of thephotolithographic mask according to the invention, for one main feature,which is oriented in a first direction at least in the region of apartial piece, there are at least four, preferably at least six assistfeatures, which, adjacent to the partial piece, are oriented in a seconddirection substantially perpendicular to the first direction.

[0025] According to a further preferred embodiment of thephotolithographic mask according to the invention, the ratio of lengthto width of the main features is greater than 2, preferably greater than3.

[0026] The invention is explained in more detail below with reference tofigures in the drawing, in which:

[0027]FIG. 1 shows a photolithographic mask in accordance with a firstembodiment of the present invention,

[0028]FIG. 2 shows a photolithographic mask in accordance with a furtherembodiment of the present invention,

[0029]FIG. 3 shows a diagram which illustrates the way in which theirradiation intensity is dependent on the defocusing of the irradiationfor the photomask shown in FIG. 2,

[0030]FIG. 4 shows a photolithographic mask in accordance with the priorart,

[0031]FIG. 5 shows a diagram which illustrates the way in which theirradiation intensity is dependent on the defocusing of the radiationfor the photo-mask shown in FIG. 4,

[0032]FIG. 6 shows a further photolithographic mask in accordance withthe prior art,

[0033]FIG. 7 shows a diagram which illustrates the way in which theradiation intensity is dependent on the defocusing of the irradiationfor the photomask shown in FIG. 6; and

[0034]FIG. 8 shows a photolithographic mask in accordance with a furtherembodiment of the present invention.

[0035]FIG. 1 shows a plan view of a photolithographic mask in accordancewith a first embodiment of the present invention. The embodiment shownin FIG. 1 has a radiation-transmissive substrate (11), for examplecomprising quartz glass, and radiation-opaque features (12, 13, 14, 15and 16) which are applied to the substrate. By way of example, blackchromium can be used as material for the radiation-opaque layer.

[0036] The mask has a group of 3 main features 12, 13 and 14, which areneither completely isolated nor arranged very close together. The mainfeatures 12, 13 and 14 are in each case provided as opaque regions onthe radiation-opaque layer. The main features 12, 13, 14 are formed insuch a way that the pattern formed by the main features 12, 13, 14 istransferred into the resist layer (not shown) when irradiated. The mainfeatures 12, 13, 14 therefore define the pattern which is to betransferred. The main features 12, 13, 14 may be regarded as individualmain features or as partial pieces of a “large”, U-shaped main feature.

[0037] The main features 12, 13, 14 shown in FIG. 1 representsubstantially isolated main features which, to improve theirreproduction properties, are dependent on additional assist features.Therefore, the assist features 16, which are oriented parallel to themain features 12, 13, 14 as in the prior art, are arranged adjacent tothe outer edges 12 a, 13 a, 14 a. The assist features 16 are rectangularin shape and are formed in such a manner that the pattern formed by theassist features 16 is not transferred into the resist layer whenirradiated.

[0038] The assist features 16 improve in particular the reproductionproperties of the outer edges 12 a, 13 a, 14 a of the main features 12,13, 14. To improve the reproduction properties of the inner edges 12 b,13 b, 14 b of the main features 12, 13, 14, assist features are onceagain required. However, the main features 12, 13, 14 are arranged insuch a way with respect to one another that conventional assistfeatures, arranged parallel to the main features 12, 13, 14, cannot beused, since these conventional assist features would interfere with oneanother. Particularly if the distance D between the two main featureslies in the range λ/(2 NA)<D<3λ/NA, it is impossible to use parallelassist features corresponding to the assist features 16.

[0039] In order, despite these difficulties, to ensure that thereproduction properties of the inner edges 12 b, 13 b, 14 b of the mainfeatures 12, 13, 14 are improved, the photolithographic mask accordingto the invention uses assist features 15 which are orientedsubstantially perpendicular to the main features 13, 14. The assistfeatures 15 are likewise of rectangular design and are arrangedsymmetrically with respect to the main features 13, 14, between the mainfeatures 13, 14.

[0040] The distance x between the assist features 15 and the mainfeatures lies in the range λ/(5 NA)<x<2λ/(3 NA), and the distance dbetween the assist features lies in the range λ/(3 NA)<d<3λ(2 NA).Furthermore, the width b of the assist features 15 lies in the rangeλ/(10 NA)<b<λ/(3 NA), where λ is the wavelength of the radiation used toirradiate the mask or the resist layer. The precise lengths, distancesand widths of the assist features 15 are dependent on a multiplicity ofprocess parameters, such as for example the photoresist used, theirradiation dose, etc., and consequently these variables have to beadjusted to match the particular lithography process.

[0041] The photolithographic mask according to the invention has theadvantage that the reproduction properties of even the edges 13 b and 14b of the main features 13 and 14 whose reproduction properties it washitherto impossible to improve by assist features are improved, with theresult that in particular the process window for the entire lithographyprocess is improved.

[0042] The embodiment of the present invention which is described withreference to FIG. 1 relates to a photo-mask which has a radiation-opaquelayer, for example a black chromium layer. The text which followsdescribes an embodiment of the present invention which, instead of aradiation-opaque layer, has a half-tone layer, for example anMoSi_(z)O_(x)N_(y) layer.

[0043] For this purpose, instead of a radiation-opaque layer, ahalf-tone layer is applied to the glass substrate, the half-tone layerbeing radiation-transmissive to a certain percentage (e.g. 3% to 10%radiation transmission) and shifting the phase of the radiation passingthrough the half-tone layer by a predetermined amount. Then, thishalf-tone layer is patterned accordingly, so that features which arematched to the pattern which is to be transferred are produced in thelayer. If the mask is then irradiated, a phase shift (generally 180°)takes place at the edges of the features, with the result that it ispossible to increase the resolution which can be achieved.

[0044]FIG. 2 shows a plan view of part of a photomask according to theinvention, in which the main features 20, 21 and 22 are formed fromhalf-tone material. The thickness of the half-tone material is selectedin such a way that a phase shift of 180° with respect to the surroundingarea takes place at the edges of the main features 20, 21 and 22 whenthe radiation passes through it. The main features 20, 21 and 22 areformed in such a way that the pattern formed by the main features 20, 21and 22 is transferred into the resist layer (not shown) when irradiated.The main features 20, 21 and 22 therefore define the pattern which is tobe transferred.

[0045] The main features 20, 21 and 22 shown in FIG. 2 in turn representsubstantially isolated main features, which are reliant on additionalassist features in order for their reproduction properties to beimproved. Accordingly, the photolithographic mask according to theinvention uses assist features 25 comprising half-tone material whichare oriented substantially perpendicular to the main features 20, 21 and22. The thickness of the half-tone material is once again selected insuch a way that, at the edges of the auxiliary features 25, when theradiation passes through it, there is in each case a phase shift of 180°with respect to the surrounding area. The assist features 25 arelikewise rectangular in form and are arranged symmetrically with respectto the main features 20, 21 and 22, between the main features 20, 21 and22. Once again, the assist features 25 are formed in such a way that thepattern formed by the assist features is not transferred into the resistlayer when irradiated.

[0046] In the present example, the main features 20, 21 and 22 have awidth of 200 nm, which, when irradiated, leads to a feature with a widthof 190 nm in the resist layer. The distance between the main features is1000 nm. The assist features 25 have a width of 50 nm and are arrangedat a distance of 125 nm. This shows that, unlike with conventionalphotomasks, in a photo-mask according to the invention the so-called“mask bias”, i.e. the widening of the feature on the mask compared tothe width of the feature in the resist layer, can be considerablyreduced.

[0047]FIG. 3 shows a diagram which represents the way in which theirradiation intensity is dependent on the defocusing of the irradiation.The area below the curve represents the combinations of parameters withwhich good reproduction of the main features can be achieved. The areabelow the curve is therefore a measure of the process window withinwhich the irradiation parameters may be varied. It can be seen that thedefocusing of the irradiation can be varied over a wide range, yet it isnevertheless possible to ensure good reproduction of the main features.

[0048] For comparison purposes, FIG. 4 shows a photomask in accordancewith the prior art, which has only the main features 20, 21 and 22 butno assist features. In this case, the width of the main features 20, 21and 22 is 250 nm, in order to generate tracks with a width of 190 nm inthe resist layer. Accordingly, the conventional photomask requires aconsiderably greater mask bias than the photomask according to theinvention. As can be seen from FIG. 5, the size of the process window isalso considerably reduced for the conventional photomask, which inpractice means that the main features 20, 21 and 22 cannot bereproducibly transferred into a resist layer.

[0049] Once again for comparison purposes, FIG. 6 shows a photomask inaccordance with the prior art which has the main features 20, 21 and 22and conventional, parallel assist features 26. In this case, the widthof the main features 20, 21 and 22 is 240 nm, in order to generatetracks with a width of 190 nm in the resist layer. Accordingly, thisconventional photomask once again requires a considerably greater maskbias than the photomask according to the invention.

[0050] Although in this example the main features are at a distance(1000 nm) at which conventional parallel assist features can be used,even for this photomask the process window is considerably reduced insize, as can be seen from FIG. 7.

[0051]FIG. 8 shows a photolithographic mask according to a furtherembodiment of the present invention. Unlike the embodiments of thephotolithographic mask according to the invention which have beendescribed hitherto, the assist features 15 are not formed as rectangles,but rather are composed of triangles. The assist features 15 once againhave the advantage that the edges 13 b and 14 b of the main features 13and 14, whose reproduction properties it was hitherto impossible toimprove by means of assist features, experience an improvement in theirreproduction properties, with the result that in particular the processwindow for the entire lithography process is improved.

1. A photolithographic mask for the exposure of radiation-sensitiveresist layers on semiconductor substrates, the mask having at least oneradiation-transmissive substrate and at least one half-tone layer, thehalf-tone layer being used to provide main features (12, 13, 14, 20, 21,22), the main features (12, 13, 14, 20, 21, 22) being formed in such away that the pattern formed by the main features (12, 13, 14, 20, 21,22) is transferred into the resist layer when irradiated, and thehalf-tone layer being used to provide assist features (15, 16, 25, 26),the assist features (15, 16, 25, 26) being formed in such a way that thepattern formed by the assist features (15, 16, 25, 26) is nottransferred into the resist layer when irradiated, characterized in thatfor one main feature (13, 14, 20, 21, 22), which is oriented in a firstdirection at least in the region of a partial piece, there are at leasttwo assist features (15, 25), which, adjacent to the partial piece, areoriented in a second direction which is substantially perpendicular tothe first direction.
 2. The photolithographic mask as claimed in claim1, characterized in that between two directly adjacent main features(13, 14, 20, 21, 22), which at least in the region of a partial pieceare oriented substantially parallel in a first direction, there are atleast two assist features (15, 25), which are oriented in a seconddirection substantially perpendicular to the first direction.
 3. Thephotolithographic mask as claimed in claim 1, characterized in that theassist features (15, 25) are rectangular in shape.
 4. Thephotolithographic mask as claimed in claim 1, characterized in that theassist features (15) are composed of triangular shapes.
 5. Thephotolithographic mask as claimed in claim 1, wherein the width b of theassist features (15) lies in the range λ/(10 NA)<b<λ/(3 NA).
 6. Thephotolithographic mask as claimed in claim 1, wherein the distance Dbetween the main features (13, 14, 20, 21, 22) lies in the range λ/(2NA)<D<3λ/NA.
 7. The photolithographic mask as claimed in claim 1,wherein the distance x of the assist features (15, 25) from the mainfeatures lies in the range λ/(5 NA)<x<2λ/(3 NA).
 8. Thephotolithographic mask as claimed in claim 1, wherein the distance dbetween the auxiliary structures (15, 25) lies in the range λ/(3NA)<d<3λ/(2 NA).
 9. The photolithographic mask as claimed in claim 1,wherein for one main feature (13, 14, 20, 21, 22), which is oriented ina first direction at least in the region of a partial piece, there areat least four assist features (15, 25), which, adjacent to the partialpiece, are oriented in a second direction substantially perpendicular tothe first direction.
 10. The photolithographic mask as claimed in claim1, wherein the ratio of length to width of the main features is greaterthan 2.